100 years ago many places in Europe and the US were "corrupt incompetent primitive societies".
Things haven't changed since then. There are still welfare-benefit sinkholes all over the US and the UK which are corrupt, incompetent primitive societies. The problem is that increased technology allows the useless elements of society to outbreed the more intelligent, and the latter are the only ones who tend to have some sense re not breeding too much. We need laws to correct this. Hmm, I always remember a wonderful story by Harry Harrison("a criminal act") wherein he posits a future, vastly overpopulated world where any man who produces more than one child can be fought, each time, in one duel to the death by any one volunteer with the latter being armed to the teeth. A logical way to reduce overpopulation.
Technology/education changes things. Every society lowers its birth rate once things start getting better, especially once women start getting college educations.
Not necessarily. There was the "baby boom" generation, for example. Current drops in population are due, imo, to government attacks on family values, feminism, drop in cultural activities etc.
As far as low-quality nutrients, the oceans hold far more mineral content than we'll ever need. For that matter, we'd be able to recycle the vast majority of the mineral content anyway, from waste matter, once we got the vat-grown food system started.
No, the ocean is too diluted, despite containing tons upon tons of such minerals. People have already tried and failed to harvest minerals from the ocean, because of this.
I don't like the notion of living off recycled crap. It reminds me of the book Dune in which people have to live off their own recycled urine which has been absorbed into their stillsuits via urination.
As far as people wiping out planets on a whim, one thing I fully expect us to be able to do (assuming Moore's Law continues for at least 25-30 more years) is to cure all mental illnesses, including impulsivity and excessive anger.
That's a bit of a stretch, to class impulsivity and excess anger as mental illness. They can be quite normal behaviour under the right circumstances. And I doubt that any society would dare try to make us all unemotionally calm types as then we'd all be like zombies.
Basically, an alien's behaviour may be so utterly alien that it could wipe out planets for the most alien of reasons, not even needing aggression or hatred to be able to commit the act. If it weren't even aware of us(say if it was an energy-being) then it could still wipe us out, unknowingly.